Electric condenser bushing



June 22, 1943. l. w. A KIRK'wooD ETAL ELECTRIC GONDENSER BUSHING Original Filed June 15, 1939 LlguLdS nL 01g/ Poli/mera zed Patented June 22, 1943 2,322,215 ELECTRIC coNnENsaa -nusnnzc 1an Ward Anderson Kirkwood' and Patrick Dunbar Ritchie, Newcastle-on-Tyne, England, assignors to A. Reyrolle & Company Limited, Hebburn-on-Tyne, England, a company oi' Great Britain ,Original application Jane 13, 1939, Serial No.

278,965. Divided and this 1942 Serial No. 440,222.

application April 23, In Great Britain June 12 claims. (c1. 174-143) This application is a division from our application, Serial No. 278,965, led June 13, 1939.

This invention relates to electric condenser bushings.

Electric insulating bushings which consist of superimposed layers of a cellulosic base, such for example as paper or cloth, with one or more stress-grading conducting foils between the layers which are bonded with synthetic resins or other binders are subject to the disadvantage that they are liable to absorb moisture, since cellulosic materials, in general, are inherently hygroscopic. Even if the cellulosic insulator is impregnated with a synthetic resin the absorption of moisture by the iibres will not be wholly prevented and there will ultimately be a decrease in the dielectric strength, increase of power factor, and swelling of the insulator at the exposed ends of the fibrous layers.

The main object of the present invention is to overcome this disadvantage by forming a condenser bushing of materials which are essentially non-hygroscopic.

A further object of the invention is to provide a condenser bushing comprising a plurality of concentric superimposed layers of fibres of a vitreous substance, at least one stress grading conducting foil between adjacent vitreous layers and a solid insulating medium iilling the interstices in the vitreous layers, the insulating medium having a dielectric strength within the interstices exceeding that of air at atmospheric pressure.

It is desirable that the vitreous substance employed beuniform in structure and free from incorporated foreign matter, that it will provide individual bres of a satisfactory textile length, and that it will be wholly (or substantially wholly) non-hygroscopic.

In order to facilitate lling of the interstices between the bres with the insulating medium the bres may be bound together in open texture by a preliminary binder prior to filling of the interstices with the insulating medium.

According to a further object of the invention the interstices in the brous layers are illled with an insulating binder, such for example as a phenol-formaldehyderesin, a higher chlorinated naphthalene, or shellac applied in solution or,

in certain cases where this is possible, in liquid or semi-liquid state. The insulating binder'may be applied to the fibres prior to the Winding or wrapping which is then effected under the application of pressure and/or heat so that the layers are impregnated with the binder which binds the v glass wool A3 is rst wrapped ample as styrene, this liquid being then polymerised by heating, if necessary under pressure. Thus, for example, the liquid impregnantA may be forced through the insulating mass from end to end so as to force out the air and i111 the interstices with the impregnant.

With a view to imparting flexibility as may be desired al suitable plasticiscr may, in any of the above methods, -be incorporated in the insulating binder. A

In the accompanying drawing Figure 1 illustrates one method of forming a condenser bushing according to the invention,

Figure 2 shows the ilnished bushing in longitudinal section,

Figure 3 illustrates another to the invention, and

- Figure 4 shows the nished tudinal section.

In Figure 1 the bushing is formed by applying to a cord or a strip of felted glass wool or woven glass cloth, for example a glasswool fabric, a binder constituted by phenol Iormaldehyde resin, polystyrene or polyvinyl acetal. The strip or cord A thus treated or varnished is dried and then wrapped in a number of superimposed layers A1 about a former B, which may be a conductor, wlththe insertion of stress-grading foils C between the layers A1 as they are wound. The

method according bushing in longilayers A1 with the foils C between them are subjected to heat and pressure by rolls D so that the interstices in and between the layers A1 are impregnated with the binderV and the insulation is thus bound into a solid mass about the former or conductor B as shown in Figure 2.

In accordance with a second method, as shown in Figure 3, a cord or a strip of felted or woven round a conductor or former Bl in a number of superimposed layers A4, stress-grading elements Cl being inserted between the layers. A4 as they are wound. The layers are then impregnated, preferably in vacuo, with a polymerisable liquid such for example as styrene or methyl methacrylate, or solutions therein of the corresponding polymers. The liquid is then polymerised by the application of heat, if necessary under pressure to avoid formation of bubbles. The finished bushing is shown in Figure 4.

According to a` modified method the cord or strip of woven or felted glass wool A (Figure 1) wound in a number of superimposed layers about a conductor or former with stress-grading foils C between them, is dipped or otherwise impregnated, preferably in vacuo, with a molten waxllke material, such for example as chlorinated naphthalene or ceresine, which on cooling will set to bind the fibres into a solid mass.

According to another modification a cord or strip of glass cloth, for example felted or woven glass wool has applied thereto shellac, a natural wax, a chlorinated naphthalene or a phenolic, urea, vinyl or acrylic resin, each in a suitable solvent which is then removed in known manner. A cord or fibrous strip thus treated is wound or wrapped in a number of superimposed layers about a conductor or former as described with reference to Figure 1, metal foils being inserted between adjacent layers as they are wound and subjected to simultaneous heat and pressure by the rolls D.

In order to impart flexibility as may be desired a suitable plasticiser, such for example as chlorinated diphenyll amyl naphthalene, dibutyl phthalate, or tricresyl phosphate, may in each of the above methods be incorporated in the binden Impregnation of the interstlces between the fibres will be facilitated if the texture of the fibrous layers ls as open as may be practicable. Though it is a matter of some difiiculty to weave a fibre-glass textile of loose open mesh owing to warp-slip, this difficulty may be overcome by using, for example in any of the methods above described, a felted mat of glass fibres held together by a small quantity, say 1% to 2% by weight, of a preliminary binder such for example as phenol-formaldehyde resin or polystyrene, the binder content being just sufficient to hold the fibres against slip at their main points of mutual contact. This gives an open-textured material which will be readily permeated by the liquid impregnant. Alternatively a felted mat as described may be employed, for example with the first and fourth methods above described, in which the preliminary binder content is high, for example 40% to 60% by weight of the whole, so that. after winding or Wrapping, impregnation of the felted mat by the binder can be effected by hot-pressing or hot-rolling the superimposed layers. If a thermoplastic binding is employed with this method careful control of the tension to which the material is subjected during winding or wrapping should be exercised in order to avoid undesirable stretching.

The felted mat, incorporating the binder, may be formed for example by agitating a suspension of short fibres of a vitreous substance (e. g., glass Y wool) in a reaction mixture giving rise to a synthetic resin. For example, the libres may be stirred into a heated mixture of phenol and aqueous formaldehyde in suitable proportions. When the condensation has proceeded to the point where the reactants have formed an A-stage phenolic resin (resole) the whole is poured out on to a smooth, endless conveyor belt, forming a thin film ,from which all surplus water is carefully evaporated, if desired under reduced pressure. The resulting film may then be wound into a bushing in the usual manner under the action of heat and pressure.

It will be understood that the electrical, mechanical, and other properties of the bushing produced in accordance with the invention may be modified to suit requirements and conditions of use, by suitable selection not only of the vitreous material but also the filleror binder. Thus, a lead glass will usually give a lower power factor and a higher dielectric constant than a sodalime glass, but both possess a surface which is somewhat susceptible to atmospheric moisture. The thickness of the layer of moisture which can be adsorbed on the surface of the fibres will depend upon their chemical composition, such thickness usually decreasing, for example, with decrease of the soda and increase of the boric oxide content of the fibres. It is in general preferred to employ the substantially alkali-free boro-silicate glass fibres already available in the industry, but a particularly desirable combinae tion of materials, from the dielectric standpoint, comprises vitreous silica fibres impregnated or bound with a hydrocarbon such as polystyrene, or a polymerised ethylene plastic. When employing woven fibres the warp may differ chemically or otherwise from the weft, so as to produce intentional differences, in two mutually perpendicular directions, of various characteristics, such for example as elasticity. When the lowest possible power-factor is required, it is desirable that the fibres be as free as possible from the emulsions of oil, starch, and other materials with which they are usually moistened during spinning into thread form. These materials usually contain emulsifying agents which, though present only in minute amount on the iinal weave, are polar compounds whichl could with advantage be removed, prior to the application of the binder or impregnant, for example by washing with water, trichlorethylene or other suitable solvents or vapours thereof, followed by intensive drying.

The vitreous substance employed in accordance with the invention has no inherent tendency to absorb moisture and, `though it may tend to adsorb a surface layer of moisture from a humid atmosphere, this layer can readily be removed by heating prior to the fibres being filled or bound to form the bushing. For this purpose, by using an inorganic vitreous substance, a higher temperature can safely be employed than with cellulosic materials which will only withstand a comparatively low temperature. Further, whereas a cellulosic material, such as paper, coated with polystyrene, can be pressed into laminated sheets at temperatures of about C. to 160U C., paper coated with polymerised N-vinyl carbazole would require a temperature of about 250 C. to 300o C. in orderto render the binder sufficiently plastic to flow readily into the interstices. This would. however, damage the paper and render it brittle. By employing an inorganic vitreous substance in stead of cellulosic material much higher manufacturing temperatures can safely be applied. Again, bushings composed of cellulosic materials bound with phenolic resins are readily tracked that is to say they produce H'a carbonised conducting track When brought into contact with an electric arc. This tendency can be reduced by substituting a urea for a phenolic resin, or glass for cellulosic material. It can be substantially eliminated in an insulating body according to the invention using glass fibres bound with a urea or other non-tracking plastic since an arc will inere. ly cause fusion of the surface iibres.

It will be understoodthat the application of the insulating ller or binder 'may be efl'ected either before or after winding or wrapping of the 5 -successive layers of the vitreous substance.- For example, the vitreous substance may be varnished with a thermo-setting varnish'beiore winding on the conductor or former, or the layers may be wound before the interstices are filled, say by i0 conductor. a series of superimpo a` monomer which is subsequently polymerised, or

a syrupy solutionY of a polymer in a monomer may\ be applied to the vitreous substance before wind-l ing the layers on the conductonr former and then polymerising.

In any o! themethods above described the filler or binder may be applied as a'n emulsion or suspension of globules or particles of the filling or binding medium in an -immiscible liquid. A

'I'he superimposed layers may be formed from 20 a textile fabric woven from spun fibres or threads of the vitreous substance. These layers m'ay also be formed by winding thin fibres, sheets, irregular'layers similar to felt, or extruded woven or braided cord, made oi the said vitreous substance. What we claim as our inventionv and desire tosecure by Letters Patent is:

. 1. An electric condenser bushing comprising a plurality of concentric superimposed `layers ofI fibres of a vitreous substance, at least one stress 30 grading conducting i'oil between adjacent vitreous layers and a solid `insulating medium illling the interstices inthe vitreous layers, the insulating medium having a dielectric strength within the interstices exceeding that or air at atmospheric 35 pressure.

2. An electric condenser bushing comprising a conductor, a strip ot glass wool wound in a number of superimposed layers about the conductor.

at least one stress-grading conducting foil be- 40 tween adjacent layers, and a resinous ller tilling the interstices in the .layers so as to bind the whole into a compact'mass about the conductor.

3. An electric condenser bushing as claimed in plasticiser.

V 3 plurality of concentric superimposed layers of woven ilbres oi a vitreous substance, at least one stress grading conducting foil between adiacent vitreous layers and a poLvmerlsed binder iilling the interstices in the layers, the binde having been polymerised inl the assembled bush g so as to bind the whole into a compact mass about the conductor. t

5. An`electric condenser bushing comprising a sed layers consistingiof a strip ot glass-wool libres wound on the conductor, at least one ystress-grading conducting foil between .adjacent layers, and a phenol-formaldehyde resin filling. the interstices 5 inthe-strip and polymerised so as to bind the whole into a compact mass about the conducton 6. An electric condenser bushing as claimed in claim 1. in which the insulating medium consists of a polymerised aromatic hydrocarbon of the vinyl type.

7. `An electric condenser bushing as claimed in claim 1, in which the insulating medium consists oi'POlyViDYl acetal.

8. An electric condenser bushing comprising a 25 conductor, a series `-of superimposed 4layers of glass-wool nbres on the conductor, at least one.

stress-grading conducting foilbetween adjacent layers, and a polymerised electrically insulating binding-material containing a plasticiser lling the interstices in the layers. e

claim, in which the `-plasticiser 'consists oi' amyl naphthalene.

10. An electric condenser bushing as claimed in claim 8, in. which the binding materiallconsists of a polymerised aromatichydrocarbon ofthe vinyl.

' nated dinhenyi4 claim 2 in which-the resinous mass contains a-45 Y.

IAN wann ANDERSON Kmkwoon. i PATRICK DUNBAR 9. An'electric condenser bushing as claimed in 

